The present invention relates generally to methods for etching the edge of a wafer and, more particularly, to methods for etching the edge of a wafer using a potassium-based chemical oxidizer, such as potassium permanganate, in the presence of hydrofluoric (HF) acid.
Wafers 10, such as silicon wafers, that form the substrate upon which semiconductor devices are formed must be made to exact specifications and must be generally free of manufacturing defects. During a typical wafer fabrication process, wafers are sliced from an ingot. The resulting wafers have relatively rough, but square edges as depicted in FIG. 1A. Thereafter, the wafer edge is ground to size the wafer to an exact diameter and to form the edges into a preferred geometric shape. In this regard, the edge is generally ground to define chamfered portions proximate each of the opposed major surfaces. See FIG. 1B. This grinding process, however, can create imperfections in the wafer (depicted schematilcally as pits 12 in FIG. 1) which, in the ensuing machining processes, can cause problems, such as by leading to mounding or crowning of a subsequently deposited epitaxial layer, as described in detail hereinbelow.
Following grinding of the wafer edge, the wafer 10 is typically cleaned in an alkaline solution and, in many instances, is then subjected to an acid etching treatment. See FIGS. 1C and 1D. The alkaline cleaning will tend to preferentially etch the wafer edge and, as such, may actually enlarge the imperfections created by the grinding process. While the acid etching may somewhat decrease the roughness of the wafer edge, the acid etching typically does not completely counteract or offset the effects of the alkaline cleaning.
Typically, wafers 10 are processed to establish gettering sites on the backside of the wafer. As known to those skilled in the art, the gettering sites attract bulk impurities, such as transition metals, away from the front surface of the wafer upon which semiconductor devices are typically formed. One technique for creating gettering sites on the backside of a wafer is to sandblast or otherwise roughen the back surface of the wafer. However, another common technique for establishing gettering sites on the backside of a wafer is to deposit a polysilicon layer 14 on the wafer, typically by means of a low pressure chemical vapor deposition (LPCVD) process. The polysilicon layer is subsequently removed, such as by polishing, from the front surface of the wafer, but remains on the back surface of the wafer in order to serve as gettering sites for bulk impurities within the wafer. Unfortunately, in depositing the polysilicon layer on the wafer, the polysilicon layer is also deposited on the wafer edge and within the imperfections in the wafer edge as shown in FIG. 1E.
As also shown in FIG. 1E, a silicon dioxide (SiO2) layer 16 is also oftentimes deposited on the back surface of the wafer 10 in order to create a back seal to prevent autodoping during subsequent processing of the wafer, thereby preventing dopants from escaping through the back surface of the wafer and being disadvantageously redeposited upon the front surface of the wafer. In particular, an SiO2 layer is typically deposited on the back surface of those wafers that will subsequently undergo an epitaxial deposition process as described hereinafter. Typically, the SiO2 layer is deposited by chemical vapor deposition (CVD) process. As a result, SiO2 layer is deposited not only on the back surface of the wafer, but also upon the wafer edge as shown in FIG. 1E. In order to remove the SiO2 from the wafer edge, the wafer is typically cleaned and the wafer edge is then etched with HF acid. While the HF acid removes the SiO2, the HF acid does not remove the polysilicon layer 14 that was previously deposited upon the wafer. As such, the wafer edge is still coated with polysilicon following the HF acid etching. See FIG. 1F.
Following the HF etching process, the wafers are then rinsed and inspected. Following inspection, the edges are subjected to chemical mechanical polishing (CMP), typically by means of an edge polishing machine, such that the resulting edge surfaces have a smooth mirror-like finish that resists the subsequent adhesion of contaminants.
See FIG. 1G. As known to those skilled in the art, edge polishing commonly utilizes a slurry which provides the abrasive for the polishing process. Since the wafer edges are still coated with polysilicon, however, the polishing process takes a substantial length of time in order to remove the polysilicon layer 14 and expose the bare silicon. As a result, the throughput of the wafer fabrication process may be slowed somewhat by the length of time required to remove the polysilicon layer during the polishing of the wafer edge. Additionally, even once the wafer edge has been polished so as to remove the polysilicon layer, polysilicon will remain in any imperfections 12 not removed from the wafer edge. Any polysilicon that remains on the contoured edge at an acute angle from the plane of the wafer""s front surface has the potential to act as an initiating site for a failure phenomena resulting from epitaxial deposition upon the site that is commonly referred to as a nodule. Effective removal of the polysilicon results in fewer initiating sites where nodules often form during subsequent epitaxial deposition as described below.
Once the wafer edge has been ground, etched, and polished, as described above, the front surface of the wafer 10 can be polished, usually by means of a polishing machine that sequentially employs slurry having particulates of decreasing sizes in order to finely polish the front surface. See FIG. 1H. While the polishing of the front surface of the wafer can complete the wafer fabrication process, some semiconductor device manufacturers desire for an epitaxial layer to be deposited upon the front surface.
During the deposition of an epitaxial layer 18 upon the front surface of the wafer 10, the epitaxial layer will build upon and continue the crystal orientation, or lack thereof, of the polysilicon in the form of nodules on the wafer edges. Since the polysilicon pre-nodule sites have an indeterminate crystal orientation, an epitaxial layer deposited upon this site will also have an indeterminate crystal orientation, thereby rendering that portion of the wafer unfit for the fabrication of most semiconductor devices due to the resulting irregular surface texture near the edge region on the front side of the wafer. Moreover, the edge region of the epitaxial layer will tend to mound or crown as depicted in FIG. 1I, thereby preventing the epitaxial layer from having a flat surface, as desired. To date, however, it has proven difficult to remove the polysilicon from the imperfections prior to growing the epitaxial layer on the front surface of the wafer.
The present invention relates to a method for fabricating a wafer that grinds the edge of the wafer to size the wafer and to shape the wafer edge and, following the deposition of a polysilicon layer on the wafer, etches the wafer edge with a potassium-based chemical oxidizer, such as potassium permanganate, in the presence of HF acid in order to remove polysilicon from and otherwise reduce the roughness of the wafer edge. The edge of the wafer is then polished in a more efficient manner and for a shorter length of time than conventional edge polishing techniques since the polysilicon layer has already been removed and the corresponding roughness of the wafer edge has been reduced in advance of the edge polishing.
The edge of the wafer is typically etched with a potassium-based chemical oxidizer in the presence of HF acid at a removal rate of between 1.5 microns/minute and 4 microns/minute. For these removal rates, the etchant typically has a ratio of HF acid to potassium-based chemical oxidizer of between 2:1 and 4:1. Not only does the potassium-based chemical oxidizer in the presence of HF acid remove polysilicon from the wafer edge, but the potassium-based chemical oxidizer in the presence of HF acid also removes any silicon dioxide that has been deposited upon the wafer edge. Further, the potassium-based chemical oxidizer in the presence of HF acid also cleans or etches the outermost surface of the bare silicon in order to reduce the roughness of the wafer edge.
The method of the present invention also contemplates covering the opposed major surfaces of the wafer while exposing the wafer edge to the potassium-based chemical oxidizer in the presence of HF acid. As such, the potassium-based chemical oxidizer and the HF acid will only etch the exposed edge and not the opposed major surfaces of the wafer. In one embodiment, for example, a plurality of wafers are stacked such that the major surface of one wafer faces a major surface of an adjacent wafer. By sandwiching the wafers about a spacer, such as a Teflon(trademark) spacer, the opposed major surfaces are therefore protected from the etchant.
According to the present invention, the method controllably etches the edge of a wafer with a potassium-based chemical oxidizer in the presence of HF acid to remove polysilicon, including polysilicon pre-nodule sites, from the wafer edge, thereby eliminating the initiating site for nodule formation on any subsequently deposited epitaxial layer and enhancing wafer edge smoothness for all wafers, both wafers having an epitaxial layer and wafers without an epitaxial layer. By removing the polysilicon from and reducing the roughness of the wafer edge, the edge can also be polished more quickly and efficiently, thereby potentially increasing the throughput of the wafer fabrication process.